1. Field of the Invention
This invention relates to signal generators and, more particularly, to clock driver circuits that supply clocking signals to other digital circuitry. The invention also relates to the generation of internal microprocessor clock signals and to methods of manufacturing clock driver circuits.
2. Description of the Relevant Art
A clock signal is a periodic waveform whose primary function is to provide timing references for controlling the activities of other digital circuitry. The clock signal is typically provided from a clock driver circuit that is designed to meet the drive requirements of the circuitry during worst case conditions.
The frequency of the clock signal is often an important consideration with respect to the operation of a digital circuit. In general, as the speed of the clock signal increases, the time required for the digital circuitry to perform its designated function decreases. For example, it is well known that microprocessor circuits require one or more internal clock signals to control various subsections of the microprocessor circuitry. In general, as the speed of the internal microprocessor clock increases, the time required to execute a particular program decreases. Thus, microprocessors having relatively high internal clock frequencies are desirable for high performance and computational intensive applications.
On the other hand, as the frequency of the clock signal increases, a stronger clock driver circuit is usually necessary to meet the drive requirements of the digital circuitry to be driven. Stronger clock driver circuits that operate at relatively high frequencies typically consume more power and are often more expensive, both in design and manufacture.
The frequency and edge rate (slew rate) of clock signals can also effect the electromagnetic interference (EMI) generated. Electromagnetic interference is generated by nearly all electrical circuits. The quantity of EMI radiated by a circuit is based upon many factors, including the transient current generated by the clocks and other logic circuits driven by the clocks. In general, the circuits on a semiconductor chip are a source of the transient currents, and the surrounding components such as the device package, the printed circuit board, and the cables attached to the printed circuit board act as antennae that radiate the high frequency components of the transient currents.
The period of a clock signal determines the fundamental frequency of the spectral envelope and the edge rate determines the amplitude of the harmonic components. In the case of an ideal square wave in comparison to an actual waveform, the actual waveform will have reduced high frequency components. Given this, a clock driver that provides clock signals with increased frequency and edge rates will typically be associated with increased EMI.
Chip manufacturers typically control EMI emissions through package design techniques. These techniques include power/ground planes within the package, grounded seal lids, and rails for by-pass capacitors. Systems manufacturers typically use board-level and enclosure techniques including moating in the PC board, separate power-ground planes, chokes, decoupling capacitors, and shielding. Many of these techniques are relatively expensive to employ.
EMI reduction is a rather important feature for manufacturers who would like to comply with FCC Class B and other requirements. Compliance with FCC Class B allows the device to be used in either a residential or a commercial application. Class A is restricted to industrial use only. Thus, a product that conforms to Class B will include a much larger market.
As a result of these tradeoffs of performance, cost, and generated EMI, manufacturers commonly provide several versions of the same type of digital circuit that meet different EMI, power and speed targets; one version that operates at a relatively high speed for high performance or computational-intensive applications, and another version that operates at a lower speed for low EMI, low power, or low cost applications. Such practice is typical within, for example, the microprocessor industry. To provide differing versions of the same microprocessor family, a manufacturer may fabricate a microprocessor with two separate clock generators incorporated on the semiconductor die. One of the clock generators is designed to have a strong drive capability to meet the drive requirements during high frequency operation, and the other is designed to have a weaker drive capability that will only meet the drive requirements during the lower frequency of operation. Metal mask programming methods can be employed during the fabrication of the microprocessor to enable one of the clock generators and disable the other clock generator. Unfortunately, this technique is somewhat expensive since a different set of masks must be used for the different microprocessor versions and, in addition, considerable die space is wasted since one of the clock generators on the semiconductor die is permanently disabled. Furthermore, once the microprocessor chip has been fabricated for use with a crystal oscillator of a predetermined maximum frequency, the internal maximum clock frequency cannot be changed following fabrication.
The microprocessor manufacturer may alternatively employ a fuse link that can be optionally blown open at wafer sort (a test operation that occurs after a semiconductor wafer has been fabricated but prior to dicing the wafer into individual circuits which will then be packaged) to enable one of the clock generators and disable the other. Likewise, a fuse may be blown after the device has been packaged by applying a voltage or current pulse to an external pin or pins on the package housing. Fuse links are a better option than metal masks from the standpoint that they can be configured during wafer sort using performance data; however, they present several other problems. Fuses pose a reliability risk in that they may, over time, repair themselves. Fuses further reduce the reliability of the device since they require openings in the passivation coating of the die. Finally, fuses require that the probe card (provided to electrically connect the semiconductor die to a test system) routinely handle large current transients in order to blow the fuse. This can add cost to the manufacturing of the device since the probe cards may require servicing more often.